Electrolysis of carbon dioxide in aqueous media to carbon monoxide and hydrogen for production of methanol

ABSTRACT

An environmentally beneficial method of producing methanol from varied sources of carbon dioxide including flue gases of fossil fuel burning power plants, industrial exhaust gases or the atmosphere itself. Converting carbon dioxide by an electrochemical reduction of carbon dioxide in a divided electrochemical cell that includes an anode in one cell compartment and a metal cathode electrode in another cell compartment that also contains an aqueous solution comprising methanol and an electrolyte of one or more alkyl ammonium halides, alkali carbonates or combinations thereof to produce therein a reaction mixture containing carbon monoxide and hydrogen which can be subsequently used to produce methanol while also producing oxygen in the cell at the anode.

This application claims the benefit of provisional application No.60/949,723 filed Jul. 13, 2007, the entire content of which is expresslyincorporated herein by reference thereto.

BACKGROUND

Hydrocarbons are essential in modern life. Hydrocarbons are used as fueland raw material in various fields, including the chemical,petrochemical, plastics, and rubber industries. Fossil fuels, such ascoal, oil and gas, are composed of hydrocarbons with varying ratios ofcarbon and hydrogen, and is non-renewably used when combusted, formingcarbon dioxide and water. Despite their wide application and highdemand, fossil fuels present a number of disadvantages, including thefinite reserve, irreversible combustion and contribution to airpollution and global warming. Considering these disadvantages, and theincreasing demand for energy, alternative sources of energy are needed.

One such alternative frequently mentioned is hydrogen, and the so-called“hydrogen economy.” Hydrogen is beneficial as a clean fuel, producingonly water when combusted. Free hydrogen, however, is not a naturalenergy source, and its generation from hydrocarbons or water is a highlyenergy-consuming process. Further, when hydrogen is produced fromhydrocarbons, any claimed benefit of hydrogen as a clean fuel isoutweighed by the fact that generation of hydrogen itself, mainly byreforming of natural gas, oil or coal to synthesis gas (“syn-gas”) amixture of CO and H₂, is far from clean. It consumes fossil fuels, witha quarter of the energy of the fuel being lost as heat. Hydrogen is alsonot a convenient energy storage medium because it is difficult andcostly to handle, store, transport and distribute. As it is extremelyvolatile and potentially explosive, hydrogen gas requires high-pressureequipment, costly and non-existent infrastructure, special materials tominimize diffusion and leakage, and extensive safety precautions toprevent explosions.

It was suggested that a more practical alternative is methanol.Methanol, CH₃OH, is the simplest liquid oxygenated hydrocarbon,differing from methane (CH₄) by a single additional oxygen atom.Methanol, also called methyl alcohol or wood alcohol, is a colorless,water-soluble liquid with a mild alcoholic odor, and is easy to storeand transport. It freezes at −97.6° C., boils at 64.6° C., and has adensity of 0.791 at 20° C.

Methanol is not only a convenient and safe way to store energy. Methanoleither can be blended with gasoline or diesel and used as fuels, forexample in internal combustion engines or electricity generators. One ofthe most efficient use of methanol is in fuel cells, particularly indirect methanol fuel cell (DMFC), in which methanol is directly oxidizedwith air to carbon dioxide and water while producing electricity.

Contrary to gasoline, which is a complex mixture of many differenthydrocarbons and additives, methanol is a single chemical compound. Itcontains about half the energy density of gasoline, meaning that twoliters of methanol provides the same energy as a liter of gasoline. Eventhough methanol's energy content is lower, it has a higher octane ratingof 100 (average of the research octane number (RON) of 107 and motoroctane number (MON) of 92), which means that the fuel/air mixture can becompressed to a smaller volume before being ignited. This allows theengine to run at a higher compression ratio (10-11 to 1 against 8-9 to 1of a gasoline engine), more efficiently than a gasoline-powered engine.Efficiency is also increased by methanol's higher “flame speed,” whichenables faster, more complete fuel combustion in the engines. Thesefactors explain the high efficiency of methanol despite its lower energydensity than gasoline. Further, to render methanol more ignitable evenunder the most frigid conditions, methanol can be mixed with gasoline,with volatile compounds (e.g., dimethyl ether), with other components orwith a device to vaporize or atomize methanol. For example, anautomotive fuel can be prepared by adding methanol to gasoline with thefuel having a minimum gasoline content of at least 15% by volume (M85fuel) so that it can readily start even in low temperature environments.Of course, any replacement of gasoline in such fuels will conserve oilresources, and the amount of methanol to add can be determined dependingupon the specific engine design.

Methanol has a latent heat of vaporization of about 3.7 times higherthan gasoline, and can absorb a significantly larger amount of heat whenpassing from liquid to gas state. This helps remove heat away from theengine and enables the use of an air-cooled radiator instead of aheavier water-cooled system. Thus, compared to a gasoline-powered car, amethanol-powered engine provides a smaller, lighter engine block,reduced cooling requirements, and better acceleration and mileagecapabilities. Methanol is also more environment-friendly than gasoline,and produces low overall emissions of air pollutants such ashydrocarbons, NO_(x), SO₂ and particulates.

Methanol is also one of the safest fuels available. Compared togasoline, methanol's physical and chemical properties significantlyreduce the risk of fire. Methanol has lower volatility, and methanolvapor must be four times more concentrated than gasoline for ignition tooccur. Even when ignited, methanol burns about four times slower thangasoline, releases heat only at one-eighth the rate of gasoline fire,and is far less likely to spread to surrounding ignitable materialsbecause of the low radiant heat output. It has been estimated by the EPAthat switching from gasoline to methanol would reduce incidence offuel-related fire by 90%. Methanol burns with a colorless flame, butadditives can solve this problem.

Methanol also provides an attractive and more environment-friendlyalternative to diesel fuel. Methanol does not produce smoke, soot, orparticulates when combusted, in contrast to diesel fuel, which generallyproduces polluting particles during combustion. Methanol also producesvery low emissions of NOx because it burns at a lower temperature thandiesel. Furthermore, methanol has a significantly higher vapor pressurecompared to diesel fuel, and the higher volatility allows easy starteven in cold weather, without producing white smoke typical of coldstart with a conventional diesel engine. If desired, additives orignition improvers, such as octyl nitrate, tetrahydrofurfuryl nitrate,peroxides or higher alkyl ethers, can be added to bring methanol'scetane rating to the level closer to diesel. Methanol can also be usedin the manufacture of biodiesel fuels by esterification of fatty acids.

Closely related and derived from methanol, and also a desirablealternative fuel is dimethyl ether. Dimethyl ether is easily obtained bymethanol dehydration. Dimethyl ether (DME, CH₃OCH₃), the simplest of allethers, is a colorless, nontoxic, non-corrosive, non-carcinogenic andenvironmentally friendly chemical that is mainly used today as anaerosol propellant in spray cans, in place of the banned CFC gases. DMEhas a boiling point of −25° C., and is a gas under ambient conditions.DME has no propensity to form peroxides unlike higher homologous ethers.DME is, however, easily handled as liquid and stored in pressurizedtanks, much like liquefied petroleum gas (LPG). The interest in dimethylether as alternative fuel lies in its high cetane rating of 55 to 60,which is much higher than that of methanol and is also higher than thecetane rating of 40 to 55 of conventional diesel fuels. The cetanerating indicates that DME can be effectively used in diesel engines.Advantageously, DME, like methanol, is clean burning, and produces nosoot particulates, black smoke or SO₂, and only very low amounts ofNO_(x) and other emissions even without after-treatment of its exhaustgas. Some of the physical and chemical properties DME, in comparison todiesel fuel, are shown in Table 1.

TABLE 1 Comparison of the physical properties of DME and diesel fuel DMEDiesel fuel Boiling point ° C. −24.9 180-360 Vapor pressure at 20° C.(bar) 5.1 — Liquid density at 20° C. (kg/m³) 668 840-890 Heating value(kcal/kg) 6,880 10,150 Cetane number  55-60 40-55 Autoignitiontemperature (° C.) 235 200-300 Flammability limits in air (vol %) 3.4-170.6-6.5

Currently, DME is exclusively produced by dehydration of methanol. Amethod for synthesizing DME directly from synthesis gas by combining themethanol synthesis and dehydration steps in a single process has alsobeen developed.

Another methanol derivative is dimethyl carbonate (DMC), which can beobtained by converting methanol with phosgene or by oxidativecarbonylation of the methanol. DMC has a high cetane rating, and can beblended into diesel fuel in a concentration up to 10%, reducing fuelviscosity and improving emissions.

Methanol and its derivatives, e.g., DME, DMC, and biodiesel, have manyexisting and potential uses. They can be used, for example, as asubstitute for gasoline and diesel fuel in ICE-powered cars with onlyminor modifications to the existing engines and fuel systems. Methanolcan also be used in fuel cells, for fuel cell vehicles (FCVs), which areconsidered to be the best alternative to ICEs in the transportationfield. DME is also a potential substitute for LNG and LPG for heatinghomes and in industrial uses.

Methanol is also useful in reforming to hydrogen. In an effort toaddress the problems associated with hydrogen storage and distribution,suggestions have been made to use liquids rich in hydrogen such asgasoline or methanol as a source of hydrogen in vehicles via an on-boardreformer. It is also considered that methanol is the safest of allmaterials available for such hydrogen production. Further, because ofthe high hydrogen content of liquid methanol, even compared to purecryogenic hydrogen (98.8 g of hydrogen in a liter of methanol at roomtemperature compared to 70.8 g in liquid hydrogen at −253° C.), methanolis an excellent carrier of hydrogen fuel. The absence of C—C bonds inmethanol, which are difficult to break, facilitates its transformationto pure hydrogen with 80 to 90% efficiency.

In contrast to a pure hydrogen-based storage system, a reformer systemis compact, containing on a volume basis more hydrogen than even liquidhydrogen, and is easy to store and handle without pressurization. Amethanol steam reformer is also advantageous in allowing operation at amuch lower temperature (250-350° C.) and for being better adapted toon-board applications. Furthermore, methanol contains no sulfur, acontaminant for fuel cells, and no nitrogen oxides are formed from amethanol reformer because of the low operating temperature. Particulatematter and NO_(x) emissions are virtually eliminated, and otheremissions are minimal. Moreover, methanol allows refueling to be asquick and easy as with gasoline or diesel fuel. Thus, an on-boardmethanol reformer enables rapid and efficient delivery of hydrogen fromliquid fuel that can be easily distributed and stored in the vehicle. Todate, methanol is the only liquid fuel that has been processed anddemonstrated on a practical scale as suitable for fuel use in a fuelcell for transportation applications.

In addition to on-board reforming, methanol also enables convenientproduction of hydrogen in fueling stations for refueling hydrogen fuelcell vehicles. A fuel cell, an electrochemical device that converts freechemical energy of fuel directly into electrical energy, provides ahighly efficient way of producing electricity via catalyticelectrochemical oxidation. For example, hydrogen and oxygen (air) arecombined in an electrochemical cell-like device to produce water andelectricity. The process is clean, with water being the only byproduct.However, because hydrogen itself must first be produced in anenergy-consuming process, by electrolysis or from a hydrocarbon source(fossil fuel) with a reformer, hydrogen fuel cells are still necessarilylimited in utility.

A system for producing high purity hydrogen has been developed by steamreforming of methanol with a highly active catalyst, which allowsoperation at a relatively low temperature (240-290° C.) and enablesflexibility in operation as well as rapid start-up and stop. Thesemethanol-to-hydrogen (MTH) units, ranging in production capacity from 50to 4000 m³ H₂ per hour, are already used in various industries,including the electronic, glass, ceramic, and food processingindustries, and provide excellent reliability, prolonged life span, andminimal maintenance. Operating at a relatively low temperature, the MTHprocess has a clear advantage over reforming of natural gas and otherhydrocarbons which must be conducted at above 600° C., because lessenergy is needed to heat methanol to the appropriate reactiontemperature.

The usefulness of methanol has led to development of other reformingprocesses, for example, a process known as oxidative steam reforming,which combines steam reforming, partial oxidation of methanol, and novelcatalyst systems. Oxidative steam reforming produces high purityhydrogen with zero or trace amounts of CO, at high methanol conversionand temperatures as low as 230° C. It has the advantage of being,contrary to steam reforming, an exothermic reaction, thereforeminimizing energy consumption. There is also autothermal reforming ofmethanol, which combines steam reforming and partial oxidation ofmethanol in a specific ratio and addresses any drawback of an exothermicreaction by producing only enough energy to sustain itself. Autothermalreforming is neither exothermic nor endothermic, and does not requireany external heating once the reaction temperature is reached. Despitethe aforementioned possibilities, hydrogen fuel cells must use highlyvolatile and flammable hydrogen or reformer systems.

U.S. Pat. No. 5,599,638 discloses a simple direct methanol fuel cell(DMFC) to address the disadvantages of hydrogen fuel cells. In contrastto a hydrogen fuel cell, the DMFC is not dependent on generation ofhydrogen by processes such as electrolysis of water or reformation ofnatural gas or hydrocarbon. The DMFC is also more cost effective becausemethanol, as a liquid fuel, does not require cooling at ambienttemperatures or costly high pressure infrastructure and can be used withexisting storage and dispensing units, unlike hydrogen fuel, whosestorage and distribution requires new infrastructure. Further, methanolhas a relatively high theoretical volumetric energy density compared toother systems such as conventional batteries and the H₂—PEM fuel cell.This is of great importance for small portable applications (cellularphones, laptop computers, etc.), for which small size and weight ofenergy unit is desired.

The DMFC offers numerous benefits in various areas, including thetransportation sector. By eliminating the need for a methanol steamreformer, the DMFC significantly reduces the cost, complexity and weightof the vehicle, and improves fuel economy. A DMFC system is alsocomparable in its simplicity to a direct hydrogen fuel cell, without thecumbersome problems of on-board hydrogen storage or hydrogen producingreformers. Because only water and CO₂ are emitted, emissions of otherpollutants (e.g., NO_(x), PM, SO₂, etc.) are eliminated. Direct methanolfuel cell vehicles are expected to be virtually zero emission vehicles(ZEV), and use of methanol fuel cell vehicles offers to nearly eliminateair pollutants from vehicles in the long term. Further, unlike ICEvehicles, the emission profile is expected to remain nearly unchangedover time. New membranes based on hydrocarbon or hydrofluorocarbonmaterials with reduced cost and crossover characteristics have beendeveloped that allow room temperature efficiency of 34%.

Methanol as indicated provides a number of important advantages astransportation fuel. Contrary to hydrogen, methanol does not require anyenergy intensive procedures for pressurization or liquefaction. Becauseit is a liquid at room temperature, it can be easily handled, stored,distributed and carried in vehicles. It can act as an ideal hydrogencarrier for fuel cell vehicles through on-board methanol reformers, andcan be used directly in DMFC vehicles.

Methanol is also an attractive source of fuel for static applications.For example, methanol can be used directly as fuel in gas turbines togenerate electric power. Gas turbines typically use natural gas or lightpetroleum distillate fractions as fuel. Compared to such fuels, methanolcan achieve higher power output and lower NO_(x) emissions because ofits lower flame temperature. Since methanol does not contain sulfur, SO₂emissions are also eliminated. Operation on methanol offers the sameflexibility as on natural gas and distillate fuels, and can be performedwith existing turbines, originally designed for natural gas or otherfossil fuels, after relatively easy modification. Methanol is also anattractive fuel since fuel-grade methanol, with lower production costthan higher purity chemical-grade methanol, can be used in turbines.Because the size and weight of a fuel cell is of less importance instatic applications than mobile applications, various fuel cells otherthan PEM fuel cells and DMFC, such as phosphoric acid, molten carbonateand solid oxide fuel cells (PAFC, MCFC, and SOFC, respectively), canalso be used.

In addition to use as fuels, methanol and methanol-derived chemicalshave other significant applications in the chemical industry. Today,methanol is one of the most important feedstock in the chemicalindustry. Most of the 32 million tons of annually produced methanol isused to manufacture a large variety of chemical products and materials,including basic chemicals such as formaldehyde, acetic acid, MTBE(although it is increasingly phased out in the U.S. for environmentalreasons), as well as various polymers, paints, adhesives, constructionmaterials, and others. Worldwide, almost 70% of methanol is used toproduce formaldehyde (38%), methyl-tert-butyl ether (MTBE, 20%) andacetic acid (11%). Methanol is also a feedstock for chloromethanes,methylamines, methyl methacrylate, and dimethyl terephthalate, amongothers. These chemical intermediates are then processed to manufactureproducts such as paints, resins, silicones, adhesives, antifreeze, andplastics. Formaldehyde, produced in large quantities from methanol, ismainly used to prepare phenol-, urea- and melamine-formaldehyde andpolyacetal resins as well as butanediol and methylene bis(4-phenylisocyanate) (MDI; MDI foam is used as insulation in refrigerators,doors, and in car dashboards and bumpers). Formaldehyde resins arepredominantly employed as an adhesive in a wide variety of applications,e.g., manufacture of particle boards, plywood and other wood panels.Examples of methanol-derived chemical products and materials are shownin FIG. 1.

In producing basic chemicals, raw material feedstock constitutestypically up to 60-70% of the manufacturing costs. The cost of feedstocktherefore plays a significant economic role. Because of its lower cost,methanol is considered a potential feedstock for processes currentlyutilizing more expensive feedstocks such as ethylene and propylene, toproduce chemicals including acetic acid, acetaldehyde, ethanol, ethyleneglycol, styrene, and ethylbenzene, and various synthetic hydrocarbonproducts. For example, direct conversion of methanol to ethanol can beachieved using a rhodium-based catalyst, which has been found to promotethe reductive carbonylation of methanol to acetaldehyde with selectivityclose to 90%, and a ruthenium catalyst, which further reducesacetaldehyde to ethanol. The possibility of producing ethylene glycolvia methanol oxidative coupling instead of the usual process usingethylene as feedstock is also pursued, and significant advances forsynthesizing ethylene glycol from dimethyl ether, obtained by methanoldehydration, have also been made.

Conversion of methanol to olefins such as ethylene and propylene, alsoknown as methanol to olefin (MTO) technology, is particularly promisingconsidering the high demand for olefin materials, especially inpolyolefin production. The MTO technology is presently a two-stepprocess, in which natural gas is converted to methanol via syn-gas andmethanol is then transformed to olefin. It is considered that methanolis first dehydrated to dimethyl ether (DME), which then reacts to formethylene and/or propylene. Small amounts of butenes, higher olefins,alkanes, and aromatics are also formed.

Various catalysts, e.g., synthetic aluminosilicate catalysts, such asZSM-5 (a zeolite developed by Mobil), silicoaluminophosphate (SAPO)molecular sieves such as SAPO-34 and SAPO-17 (UOP), as well asbi-functional supported acid-base catalysts such as tungsten oxide overalumina (WO₃/Al₂O₃), have been found to be active in converting methanolto ethylene and propylene at a temperature between 250 and 350° C. Thetype and amount of the end product depend on the type of the catalystand the MTO process used. Depending on the operating conditions, theweight ratio of propylene to ethylene can be modified between about 0.77and 1.33, allowing considerable flexibility. For example, when usingSAPO-34 according to an MTO process developed by UOP and Norsk Hydro,methanol is converted to ethylene and propylene at more than 80%selectivity, and also to butene, a valuable starting material for anumber of products, at about 10%. While using an MTO process developedby Lurgi with ZSM-5 catalysts, mostly propylene is produced at yieldsabove 70%. A process developed by ExxonMobil, with ZSM-5 catalyst,produces hydrocarbons in the gasoline and/or distillate range atselectivity greater than 95%.

There is also a methanol to gasoline (MTG) process, in which medium-porezeolites with considerable acidity, e.g., ZSM-5, are used as catalysts.In this process, methanol is first dehydrated to an equilibrium mixtureof dimethyl ether, methanol and water over a catalyst, and this mixtureis then converted to light olefins, primarily ethylene and propylene.The light olefins can undergo further transformations to higher olefins,C₃-C₆ alkanes, and C₆-C₁₀ aromatics such as toluene, xylenes, andtrimethylbenzene.

With decreasing oil and gas reserves, it is inevitable that synthetichydrocarbons would play a major role. Thus, methanol-based synthetichydrocarbons and chemicals available through MTG and MTO processes willassume increasing importance in replacing oil and gas-based materials.The listed uses of methanol is only illustrative and not limiting.

Methanol can also be used as a source of single cell proteins. A singlecell protein (SCP) refers to a protein produced by a microorganism,which degrades hydrocarbon substrates while gaining energy. The proteincontent depends on the type of microorganism, e.g., bacteria, yeast,mold, etc. The SCP has many uses, including uses as food and animalfeed.

Considering the numerous uses of methanol, it is clearly desirable tohave improved and efficient methods of producing methanol. Currently,methanol is almost exclusively made from synthesis gas obtained fromincomplete combustion (or catalytic reforming) of fossil fuel, mainlynatural gas (methane) and coal.

Methanol can also be made from renewable biomass, but such methanolproduction also involves syn-gas and may not be energetically favorableand limited in terms of scale. As used herein, the term “biomass”includes any type of plant or animal material, i.e., materials producedby a life form, including wood and wood wastes, agricultural crops andtheir waste byproducts, municipal solid waste, animal waste, aquaticplants, and algae. The method of transforming biomass to methanol issimilar to the method of producing methanol from coal, and requiresgasification of biomass to syn-gas, followed by methanol synthesis bythe same processes used with fossil fuel. Use of biomass also presentsother disadvantages, such as low energy density and high cost ofcollecting and transporting bulky biomass. Although recent improvementsinvolving the use of “biocrude,” black liquid obtained from fastpyrolysis of biomass, is somewhat promising, more development is neededfor commercial application of biocrude.

The presently existing method of producing methanol involves syn-gas.Syn-gas is a mixture of hydrogen, carbon monoxide and carbon dioxide,and produces methanol over a heterogeneous catalyst according to thefollowing equations:

The first two reactions are exothermic with heat of reaction equal to−21.7 kcal.mol⁻¹ and −9.8 kcal.mol⁻¹, respectively, and result in adecrease in volume. Conversion to methanol is favored by increasing thepressure and decreasing the temperature according to Le Chatelier'sprinciple. The third equation describes the endothermic reverse watergas shift reaction (RWGSR). Carbon monoxide produced in the thirdreaction can further react with hydrogen to produce methanol. The secondreaction is simply the sum of the first and the third reactions. Each ofthese reactions is reversible, and is therefore limited by thermodynamicequilibrium under the reaction conditions, e.g., temperature, pressureand composition of the syn-gas.

Synthesis gas for methanol production can be obtained by reforming orpartial oxidation of any carbonaceous material, such as coal, coke,natural gas, petroleum, heavy oil, and asphalt. The composition ofsyn-gas is generally characterized by the stoichiometric number S,corresponding to the equation shown below.

$S = \frac{( {{{moles}\mspace{14mu} H_{2}} - {{moles}\mspace{14mu} {CO}_{2}}} )}{( {{{moles}\mspace{14mu} {CO}} + {{moles}\mspace{14mu} {CO}_{2}}} )}$

Ideally, S should be equal to or slightly above 2. A value above 2indicates excess hydrogen, while a value below 2 indicates relativehydrogen deficiency. Reforming of feedstock having a higher W/C ratio,such as propane, butane or naphthas, leads to S values in the vicinityof 2, ideal for conversion to methanol. When coal or methane is used,however, additional treatment is required to obtain an optimal S value.Synthesis gas from coal requires treatment to avoid formation ofundesired byproducts. Steam reforming of methane yields syn-gas with astoichiometric number of 2.8 to 3.0, and requires lowering the S valuecloser to 2 by adding CO₂ or using excess hydrogen in some other processsuch as ammonia synthesis. However, natural gas is still the preferredfeedstock for methanol production because it offers high hydrogencontent and, additionally, the lowest energy consumption, capitalinvestment and operating costs. Natural gas also contains fewerimpurities such as sulfur, halogenated compounds, and metals which maypoison the catalysts used in the process.

The existing processes invariably employ extremely active and selectivecopper-based catalysts, differing only in the reactor design andcatalyst arrangement. Because only part of syn-gas is converted tomethanol after passing over the catalyst, the remaining syn-gas isrecycled after separation of methanol and water. There is also a morerecently developed liquid phase process for methanol production, duringwhich syn-gas is bubbled into liquid. Although the existing processeshave methanol selectivity greater than 99% and energy efficiency above70%, crude methanol leaving the reactor still contains water and otherimpurities, such as dissolved gas (e.g., methane, CO, and CO₂), dimethylether, methyl formate, acetone, higher alcohols (ethanol, propanol,butanol), and long-chain hydrocarbons. Commercially, methanol isavailable in three grades of purity: fuel grade, “A” grade, generallyused as a solvent, and “AA” or chemical grade. Chemical grade has thehighest purity with a methanol content exceeding 99.85% and is thestandard generally observed in the industry for methanol production. Thesyn-gas generation and purification steps are critical in the existingprocesses, and the end result would largely depend on the nature andpurity of the feedstock. To achieve the desired level of purity,methanol produced by the existing processes is usually purified bysufficient distillation. Another major disadvantage of the existingprocess for producing methanol through syn-gas is the energy requirementof the first highly endothermic steam reforming step. The process isalso inefficient because it involves transformation of methane in anoxidative reaction to carbon monoxide (and some CO₂), which in turn mustbe reduced to methanol.

It is clearly desirable and maybe advantageous to produce methanolwithout first producing syn-gas. It would be further advantageous to usean abundant, practically unlimited resource such as carbon dioxide asthe carbon source to produce methanol. For example, U.S. Pat. No.5,928,806, the entire content of which is incorporated herein byreference thereto, discloses production of methanol, and relatedoxygenates and hydrocarbons, based on a carbon dioxide-basedregenerative fuel cell concept.

When hydrocarbons are burned they produce carbon dioxide and water. Itis clearly of great significance, if this process can be reversed and anefficient and economic process can be found to produce methanol fromcarbon dioxide and water to be subsequently used for energy storage,fuels and production of synthetic hydrocarbons. In plant photosynthesis,carbon dioxide is captured from the air and converted with water andsolar energy into new plant life. Conversion of plant life into fossilfuel, however, is a very long process. Thus, it is highly desirable todevelop a process for chemical recycling carbon dioxide to producehydrocarbons in a short, commercially feasible time scale.

Carbon dioxide is known to be photochemically or electrochemicallyreadily reduced to formic acid with formaldehyde and methanol beingformed in only smaller amounts. Direct electrochemical reduction of CO₂into methanol under pressure also provides methyl formate. Catalytichydrogenation of carbon dioxide using heterogeneous catalysts providesmethanol together with water as well as formic acid and formaldehyde. Asthe generation of needed hydrogen is highly energy consuming, theproduction of methanol with equimolar amount of water as well as otherside products from carbon dioxide is not practical. No efficient waysfor the selective high yield, high selectivity economical conversion ofcarbon dioxide to methanol is presently known. The high selectivitylaboratory reduction of carbon dioxide to methanol with complex metalhydrides, such as lithium aluminum hydride is extremely costly andtherefore not suited for the bulk production of methanol.

Attempts have been made to chemically convert CO₂ to methanol andsubsequently to hydrocarbons by catalytic or electrochemicalhydrogenation. Catalysts based on metals and their oxides, in particularcopper and zinc, have been developed for this process. These catalystsare unexpectedly similar to the ones currently used for the conventionalmethanol production via syn-gas. It is now understood that methanol ismost probably formed almost exclusively by hydrogenation of CO₂contained in syn-gas on the surface of the catalyst. To be converted tomethanol, CO present in the syn-gas first undergoes a water gas shiftreaction to form CO₂ and H₂, and the CO₂ then reacts with hydrogen toproduce methanol. One of the limiting factors for large scale use ofsuch methanol conversion process is the availability of the feedstock,i.e., CO₂ and H₂. While CO₂ can be obtained relatively easily in largeamounts from various industrial exhausts, hydrogen is mainly producedfrom non-renewable fossil fuel-based syn-gas and therefore has limitedavailability. Further, generation of hydrogen from fossil fuels has ahigh energy requirement.

Other methods for hydrogen production from fossil fuel have beeninvestigated, including the “Carnol” process, in which thermaldecomposition of methane produces hydrogen and solid carbon. Thegenerated hydrogen is then reacted with CO₂ to produce methanol. Thisprocess is advantageous over methane steam reforming for requiringrelatively less energy, about 9 kcal for producing one mole of hydrogen,and for producing a byproduct that can be more easily handled, storedand used, compared to CO₂ emissions generated by methane steam reformingor partial oxidation. However, the thermal decomposition of methanerequires heating it to temperatures of above 800° C. and gives onlyrelatively low yield of hydrogen. The process, in any case, requiressubstantial development for commercial application.

U.S. Publication No. 2006/0235091 describes that carbon dioxide can beused in the dry catalytic reforming of methane, if natural gas isavailable, producing carbon monoxide and hydrogen to be used to producemethanol.

A publication in 1991 also report that the electrochemical reduction ofcarbon dioxide in methanol solution under pressure was found to providea high yield of methyl formate.

The methyl formate can be subsequently hydrogenatively convertedexclusively to methanol. Formic acid can be used as the hydrogen sourcefor the reduction of methyl formate to methanol over noble metalcatalysts.

Otherwise, hydrogen used in catalytic hydrogenation can be obtained fromany suitable source, such as electrolysis of water, using any suitablemethod and source of energy, e.g., atomic, solar, wind, geothermal, etc.Photolytic, thermal, enzymatic, and other means of cleavage of water tohydrogen is also possible.

In the above-described processes, a hydrogen source must be added to thereaction mixture for conversion to methanol. If methanol could beproduced on a large scale directly from electrochemical reduction ofcarbon dioxide, without the extra step of adding a hydrogen source, sucha process would be advantageous considering the abundant supply ofcarbon dioxide in the atmosphere and in industrial exhausts of fossilfuel power burning power plants and cement plants. It would at the sametime also mitigate greenhouse effect that is causing the global climatechange (i.e., global warming). The present invention now provides such aprocess to obtain these benefits.

SUMMARY OF THE INVENTION

The invention relates to various embodiments of an environmentallybeneficial method for producing methanol by reductive conversion of anavailable source of carbon dioxide including flue gases of fossil fuelburning power plants, industrial exhaust gases or the atmosphere itself.The method includes electrochemically reducing the carbon dioxide in adivided electrochemical cell that includes an anode in one cellcompartment and a metal cathode electrode in another cell compartmentthat also contains an aqueous solution or aqueous methanolic solutionand an electrolyte of one or more alkyl ammonium halides, alkalicarbonates or combinations thereof to produce therein a reaction mixturecontaining carbon monoxide and hydrogen which can be subsequently usedto produce methanol while also producing oxygen in the cell at theanode.

The alkyl ammonium halides include multi-alkyl ammonium halides andpreferably tetrabutylammonium halides. In another embodiment, thetetrabutylammonium halide is selected from the group consisting oftetrabutylammonium bromide, tetrabutylammonium chloride,tetrabutylammonium iodide or mixtures thereof. The alkali carbonatesinclude bicarbonates such as sodium or potassium bicarbonates and thelike.

While the electrode may be chosen from any suitable metal electrode,such as Cu, Au, Ag, Zn, Pd, Ga, Ni, Hg, In, Sn, Cd, Ti, Pb, and Pt,preferably the metal electrode is a gold electrode. The metal electrodeacts as a catalyst for the electrochemical reduction.

In the embodiment, the electrochemical reduction includes applying avoltage of about −1.5 to −4 V with respect to a Ag/AgCl electrode toproduce the reaction.

Advantageously, the carbon dioxide used in the reaction is obtained froman exhaust stream from fossil fuel burning power or industrial plants,from geothermal or natural gas wells. The available carbon dioxide,however, may also be obtained from the atmosphere by absorbingatmospheric carbon dioxide onto a suitable adsorbent followed bytreating the adsorbent to release the adsorbed carbon dioxide therefrom.In this embodiment, the adsorbent is treated by sufficient heating torelease the adsorbed carbon dioxide, or may also be treated bysubjecting the adsorbent to sufficient reduced pressure to release theadsorbed carbon dioxide.

The electrical energy for the electrochemical reducing of carbon dioxidecan come from a conventional energy source, including nuclear andalternatives (hydroelectric, wind, solar power, geothermal, etc.).

BRIEF DESCRIPTION OF THE DRAWINGS

The benefits of the invention will become more evident from review ofthe following detailed description of illustrative embodiments and theaccompanying drawings, wherein:

FIG. 1 shows known examples of methanol-derived chemical products andmaterials; and

FIG. 2 schematically illustrates the METHANOL ECONOMY™ process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention relates to the simple, efficient, and economicalconversion of carbon dioxide from flue gases of fossil fuel burningpower plants, industrial exhaust gases, carbon dioxide accompanyingnatural gas, carbon dioxide accompanying steam from geothermal wells orfrom the atmosphere itself to methanol, with subsequent application forenergy storage and transportation fuels, conversion to synthetichydrocarbons and its products. The carbon dioxide to methanol conversionis a better alternative to sequestration making it a renewable generalcarbon source for fuels, synthetic hydrocarbons and their products. Theuse of this process of converting carbon dioxide to methanol and itsproducts will also lead to a significant reduction of carbon dioxide, amajor greenhouse gas, in the atmosphere thus mitigating global warming.

Carbon dioxide is preferably obtained from concentrated point sources ofits generation prior to its release into the atmosphere. Carbon dioxidecan, however, also be obtained by separating atmospheric carbon dioxidewith a suitable adsorbent followed by desorption treatment to releasethe adsorbed carbon dioxide therefrom, as disclosed in PCT ApplicationNo. WO 2008/021700. This can be achieved by heating to release theadsorbed carbon dioxide, by treating it under reduced pressure or by asuitable combination of both.

Methanol produced according to the discussed processes can be used forany purpose, such as for energy storage and transportation, as a fuel ininternal combustion engines or fuel cells, to produce related fuels(dimethyl ether, by dehydration), dimethyl carbonate (by oxidativecarbonylation), to produce ethylene, propylene, higher olefins,synthetic hydrocarbons and all their derived products including and notlimited to single cell proteins.

High concentration carbon dioxide sources are those frequentlyaccompanying natural gas in amounts of 5 to 50%, those from flue gasesof fossil fuel (coal, natural gas, oil, etc.) burning power plants,exhaust of cement plants and other industrial sources. Certaingeothermal steam also contains significant amounts of CO₂.

It has now been discovered that the use of electrochemical reduction ofcarbon dioxide (CO₂), tailored over certain cathode electrocatalystsproduces carbon monoxide (CO) and hydrogen gas (H₂) in a high yieldingratio of approximately 1:2. The ratio can be between 1:2 and 1:2.1 with1:2.05 being optimal regarding efficiency and reactant cost.Electrochemical reduction of CO₂ on metal electrodes such as Cu, Au, Ag,Zn, Pd, Ga, Ni, Hg, In, Sn, Cd, Tl, Pb, and Pt can give either methylformate or CO using a variety of electrolytes and solvents (Y. Hori, H.Wakabe, T. Tsuamoto and O. Koga, Electrochimica Acta, 1994, 39,1833-1839). The gold (Au) electrode has been found particularlyeffective for the production of CO.

It has further been discovered that the electrochemical reduction of CO₂using nobel metal, preferentially a gold electrode as a catalyst inaqueous methanol (or in water) with tetrabutylammonium halides andalkali carbonates as electrolytes not only gives CO but also H₂ at thecathode, while producing oxygen gas (O₂) at the anode. Suitabletetrabutylammonium halides for use in the present invention includetetrabutylammonium bromide, tetrabutylammonium chloride, andtetrabutylammonium iodide. Tetraalkyl ammonium salts are known topromote one electron reduction of CO₂.

CO₂+2H₂O→CO+2H₂ (at the cathode) and 3/2 O₂ (at the anode)

The CO and H₂ produced at the cathode are subsequently reacted over Cuand Ni based catalysts to produce high yields of methanol (CH₃OH).

CO+2H₂CH₃OH

The specific conditions for the above-described chemical reactions aregenerally known to skilled chemists and optimum conditions can bereadily established for the reactions. Typical yields are about 60 to100%, based on the amount of CO₂, preferably about 75 to 90%, and morepreferably about 85 to 95%. At a proper voltage, i.e. about −1.5 to −4 Vwith respect to an Ag⁰/AgCl electrode, a ratio of about 1:2 of CO and H₂can be produced with good columbic efficiency at the cathode.

The electrochemical reduction of CO₂ can also be achieved efficientlyusing KHCO₃ as the electrolyte in aqueous medium. CO₂ is readily reducedin the aqueous medium over gold electrode to an optimal 1:2 (CO to H₂)ratio at the cathode at −3.2V. The columbic efficiences are quite highreaching 100%. Pure oxygen is produced at the anode. The electricityneeded for the electrochemical reduction can come from any sourceincluding nuclear or alternative energy (hydro, wind, solar, geothermal,etc.).

The present invention advantageously produces methanol without the needof adding extra reactants, such as a hydrogen source. There is also noneed to separate the product mixture in a subsequent treatment step,thereby streamlining methanol production.

The use of carbon dioxide based methanol is highly desirable as it canmitigate and eventually replace the world's reliance on fossil fuels. Inaddition, the reduction in carbon dioxide emissions as well as theremoval of excess carbon dioxide from the atmosphere will assist inreducing global warming and restoring atmospheric conditions to apreindustrial levels, thus preserving the planet's climate for futuregenerations.

CO₂ emissions from fossil fuel burning power plants and variedindustries including geothermal wells can be captured on-site.Separation of CO₂ from such exhausts is well-developed. The capture anduse of existing atmospheric CO₂ allows chemical recycling of CO₂ as arenewable and unlimited source of carbon. CO₂ absorption facilities canbe placed proximate to a hydrogen production site to enable subsequentmethanol synthesis. When the processes of the invention utilize carbondioxide from the atmosphere, the carbon dioxide can be separated andabsorbed by using various processes as described in published PCTApplication No. WO 2008/021700 and U.S. Pat. No. 7,378,561 or can berecycled chemically as described in published US Patent Application Nos.2006/0235091 and 2007/0254969. Although the CO₂ content in theatmosphere is low (only 0.037%), the atmosphere offers an abundant andunlimited supply because CO₂ is recycled. For using atmospheric carbondioxide efficiently, CO₂ absorption facilities are needed. This can beaddressed by using efficient CO₂ absorbents such as polyethyleneimines,polyvinylpyridines, polyvinylpyrroles, etc., on suitable solid carriers(e.g., active carbon, polymer, silica or alumina), which allowabsorbtion of even the low concentration of atmospheric CO₂. CO₂ canalso be captured using basic absorbents such as calcium hydroxide(Ca(OH)₂) and potassium hydroxide (KOH), which react with CO₂ to formcalcium carbonate (CaCO₃) and potassium carbonate (K₂CO₃), respectively.CO₂ absorption is an exothermic reaction, which liberates heat, and isreadily achieved by contacting CO₂ with an appropriate base. Aftercapture, CO₂ is recovered from the absorbent by desorption, throughheating, vacuum (or reduced pressure) or electrochemical treatment.Calcium carbonate, for example, is thermally calcinated to releasecarbon dioxide. As desorption is an endothermic, energy-demanding step,the appropriate treatment can be chosen to optimize absorption anddesorption with the lowest possible energy input. Thus, CO₂ can berecycled by operation of absorbing-desorbing columns in convenientcycles with modest heating and/or under reduced pressure to causedesorption of CO₂ to take place.

When methanol, methanol-derived fuels or synthetic hydrocarbons arecombusted (oxidatively used), they release CO₂ and water, thus providingthe basis methanol cycle, the artificial version of the naturalrecycling of CO₂ through photosynthesis. In contrast to the nonrenewablefossil fuel sources such as oil, gas, and coal, recycling carbon dioxidefrom industrial and natural sources to produce methanol not onlyaddresses the problem of diminishing fossil fuel resources, but alsohelps alleviate global warming due to greenhouse effect.

The effective electrochemical hydrogenative recycling of carbon dioxidedisclosed herein provides new methods of producing methanol in animproved, efficient, and environmentally beneficial way, whilemitigating CO₂ caused climate change (global warming). The use ofmethanol as a energy storage and transportation material eliminates manydifficulties of using hydrogen for such purposes. The safety andversatility of methanol makes the disclosed recycling of carbon dioxidefurther desirable.

As known in the art, methanol can be easily treated to produce variedderived compounds including dimethyl ether, produced by dehydration ofmethanol, and dimethyl carbonate, produced by reaction of the methanolby oxidative carbonylation. Methanol and methanol-derived compounds,e.g., DME and DMC as oxygenated additives, can be blended with gasolineand used in internal combustion engines with only minor modifications.For example, methanol can be added to gasoline up to 85% by volume toprepare M85 fuel. Methanol can also be used to generate electricity infuel cells, by either first catalytically reforming methanol to H₂ andCO or by reacting methanol directly with air in a direct methanol fuelcell (DMFC). DMFC greatly simplifies the fuel cell technology and makesit readily available to a wide range of applications, including portablemobile electronic devices and electricity generators.

In addition to being a conveniently storable energy source and fuel,methanol and methanol-derived DME and DMC are useful starting materialsfor various chemicals such as formaldehyde, acetic acid, and a number ofother products including polymers, paints, adhesives, constructionmaterials, synthetic chemicals, pharmaceuticals, and single cellproteins.

Methanol and/or dimethyl ether can also be conveniently converted in asingle catalytic step to ethylene and/or propylene (e.g., in a methanolto olefin or “MTO” process), the building blocks for producing synthetichydrocarbons and their products. This means that the hydrocarbon fuelsand products currently derived from oil and natural gas can be obtainedfrom methanol, which itself can advantageously be obtained from simplechemical recycling of atmospheric or industrial CO₂ sources. Anotherutilization of methanol is its ready conversion to ethanol via hydrationof derived ethylene. Many further applications are known and can beapplied to carbon dioxide derived methanol. It should be emphasized thatthere is no preference for any particular energy source needed forproducing methanol. All sources, including alternative sources andatomic energy can be used. Energy once produced must be, however, storedand transported, for which methanol is well suited.

The improved and efficient selective conversion of carbon dioxide, whichcan be from atmospheric or industrial exhaust sources, to methanolaccording to the present invention also provides the needed raw materialfor what the inventors have termed the METHANOL ECONOMY™ process. Thisallows convenient storage and transport of energy in a liquid productthat can be used as a fuel in internal combustion engines or in fuelcells and as a starting material for synthetic hydrocarbons and theirvaried products. The METHANOL ECONOMY™ process is based on the efficientdirect conversion of still available natural gas resources to methanolor dimethyl ether, as disclosed in U.S. Publications Nos. 2006/0235088and 2006/0235091, and 2007/0254969 as well as the presently disclosedreductive chemical conversion of carbon dioxide. The concept of theMETHANOL ECONOMY™ process presents significant advantages andpossibilities. In the METHANOL ECONOMY™ process, methanol is used as (1)convenient energy storage medium, which allows convenient and safestorage and handling; (2) readily transported and dispensed fuel,including for methanol fuel cells; and (3) feedstock for synthetichydrocarbons and their products currently obtained from oil and gasresources, including polymers and even single cell proteins, which canbe used for animal feed or human consumption. The environmental benefitsobtained by disclosed chemical recycling of carbon dioxide results inmitigating the global warming to ensure the well being of futuregenerations.

As methanol is readily dehydrated to dimethyl ether, the disclosedconversion of carbon dioxide to methanol is also adaptable to producedimethyl ether for fuel and chemical applications as previously noted.

The disclosed new efficient production of methanol from industrial ornatural carbon dioxide sources, or even from the air itself, providesthe needed raw material for replacing the diminishing fossil fuelthrough the METHANOL ECONOMY™ process. The conversion of carbon dioxideto methanol necessitates significant energy, which can be, however,provided by any energy source including offpeak electric power of fossilfuel (e.g., coal) burning power plants, atomic energy or any alternativeenergy sources (solar, wind, geothermal, hydro, etc.). The reduction ofCO₂ to methanol allows storage and transportation of energy in aconvenient liquid product (i.e., methanol) more convenient, economicaland safe than volatile hydrogen gas. Methanol and/or dimethyl ether areefficient fuels in internal combustion engines or in direct oxidationmethanol fuel cells (DMFC as well as raw materials for olefins,synthetic hydrocarbons and varied products). The present inventiongreatly extends the scope of the utilization of carbon dioxide for theproduction of methanol and/or dimethyl ether from natural or industrialsources, even from the air itself.

EXAMPLES

The following examples illustrate the most preferred embodiments of theinvention without limiting it.

Example 1

In a divided electrochemical cell, using tetrabutylammonium halides,preferentially tetrabutylammonium bromide as the electrolyte over goldelectrode (cathode) in aqueous methanol medium at either −1.5V or −4Vvs. Ag/AgCl reference electrode, CO₂ is reduced and water iselectrolyzed to an optimal 1:2 mixture of CO and H₂ at the cathode. Pureoxygen as well as some bromine is produced at the anode.

Example 2

In a divided electrochemical cell, using, aqueous 0.1M KHCO₃ as theelectrolyte CO₂ is reduced at the gold cathode at −3.2V vs. Ag/AgClreference electrode CO₂ is reduced and water is electrolyzed to anoptimal 1:2 mixture of CO and H₂ suitable for methanol synthesis. Thetotal faradaic efficiences for CO and H₂ production add up to 100%. Pureoxygen is produced at the anode.

1. A method of producing methanol by reductive conversion of anyavailable source of carbon dioxide, which comprises electrochemicallyreducing the carbon dioxide in a divided electrochemical cell comprisingan anode in one cell compartment and a metal cathode electrode inanother cell compartment that also contains an aqueous solution oraqueous methanolic solution of an electrolyte of one or more alkylammonium halides, alkali carbonates or combinations thereof to producetherein a reaction mixture containing carbon monoxide and hydrogen whichcan be subsequently used to produce methanol while also producing oxygenin the cell at the anode.
 2. The method of claim 1 wherein the carbonmonoxide and hydrogen gas are obtained in the reaction mixture in aratio of at least about 1:2 or with excess amounts of hydrogen gas. 3.The method of claim 1 which further comprises reacting carbon monoxideand hydrogen from the reaction mixture to produce methanol, wherein thecarbon monoxide and hydrogen gas are present in the reaction mixture ina ratio of 1:2 to 1:2.1.
 4. The method of claim 1, wherein theelectrolyte comprises one or more multi-alkyl ammonium halides, one ormore alkali carbonates or bicarbonates and methanol or water.
 5. Themethod of claim 4, wherein the multi-alkyl ammonium halide(s) includetetrabutylammonium halide(s).
 6. The method of claim 5, wherein thetetrabutylammonium halide(s) are selected from the group consisting oftetrabutylammonium bromide, tetrabutylammonium chloride,tetrabutylammonium iodide or mixtures thereof.
 7. The method of claim 1,wherein the alkali carbonates include sodium or potassium bicarbonates.8. The method of claim 1, wherein the metal electrode is a Cu, Au, Ag,Zn, Pd, Ga, Ni, Hg, In, Sn, Cd, Tl, Pb or Pt electrode.
 9. The method ofclaim 8, wherein the metal electrode is a gold electrode.
 10. The methodof claim 1, wherein the electrochemical reduction includes applying avoltage of from −1.5 to −4 V with respect to a Ag/AgCl referenceelectrode.
 11. The method of claim 1, which further comprises obtainingthe carbon dioxide from an exhaust stream from a fossil fuel burningpower or industrial plants, from a source accompanying natural gas orfrom geothermal wells.
 12. The method of claim 1, which furthercomprises obtaining the carbon dioxide source from the atmosphere byabsorbing atmospheric carbon dioxide onto a suitable adsorbent followedby treating the adsorbent to release the adsorbed carbon dioxidetherefrom.
 13. The method of claim 12, wherein the adsorbent is treatedby sufficient heating or by subjecting the adsorbent to sufficientreduced pressure to release the adsorbed carbon dioxide.
 14. The methodof claim 1, wherein electrical energy for the electrochemical reductionof carbon dioxide is provided from a conventional energy source based onnuclear, hydroelectric, wind, geothermal or solar power.